Present day pilot operated safety relief valves (POSRV's) are generally "pop type". The "pop type" POSRV consists of a main valve and a pilot relief valve. The "pop type" pilot valve opens at a predetermined pressure, thus causing the main valve to open. Maximum fluid relieving capacity occurs instantaneously. Because of safety relief valve characteristics, the pressures at which the main valve closes is always less than the relieving set pressure. If the closing pressure becomes too close to the relieving set pressure, damage can result to the main valve seat.
The pressure difference between the opening and closing point of the main valve is referred to as blowdown In the petrochemical industry, blowdown must be kept to a minimum for economical operation. Therefore, pop type POSRV's which will discharge extremely large quantities of product when called upon to do so could be replaced by modulating POSRV's. Unlike the "pop type" POSRV described above, this invention will allow the main valve to relieve sufficient quantities of fluid media according to the degree of overpressure. In essence, the main valve disc will assume a position in proportion to the percent of overpressure. In a typical POSRV, the dome pressure (that pressure which holds the main valve closed) is reduced to atmospheric pressure almost instantly, allowing the main disc to assume a full open position. Closure occurs in the same manner. The main valve closes by the pilot instantaneously pressurizing the dome.
With this invention, the pilot will allow the dome to remain pressurized at the time the main valve cracks open. Hence, closing pressure will be virtually the same as the opening pressure.